bhyve: Add arm64 support to the gdb stub

- Add -G to the arm64 getopt handler.
- Add static register definitions and extensible XML definitions.
- Provide definitions for MD bits such as breakpoint encoding and
length.
- Ensure that bhyve re-injects breakpoint exceptions that it is not
responsible for.

Reviewed by: andrew
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D44740

bhyve: Implement a PL031 RTC on arm64

Unlike amd64's, this RTC is implemented entirely in userspace. This is
the same RTC as is provided by QEMU's virt machine.

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD

bhyve: Add bhyverun and vmexit handlers for arm64

Reviewed by: corvink, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D41006